settings: add const qualifier for unmodified data source

Code using this API to set a key to a value that is a string literal
produces errors in C++ because the passed pointer is not const
qualified, allowing the possibility that the callee modifies the
referenced string literal memory.  Add the required const qualifiers
since the set function does not change the passed to it.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>